The present invention is directed to a dual input power supply which is similar to the power supply described in U.S. Pat. No. 4,780,805, but which features a more straight-forward and simpler circuit. The power supply disclosed in U.S. Pat. No. 4,780,805 maintains a substantially constant output voltage when the power supply is connected to either a low or high level of AC voltage input. This type of power supply has particular utility with electronic equipment, such as computers, which are designed for use in more than one facility where different levels of alternating current power are provided. The most common levels encountered are 115 volts and 230 volts AC. The equipment energized by the power supply requires that the DC voltage of the power supply be maintained at a substantially constant level regardless of whether the input AC voltage is 115 volts or 230 volts. The most commonly used prior art method for achieving the foregoing is to switch the power supply from a full-wave rectifier operation to a voltage doubler operation. Various techniques have been used in the past to accomplish such switching, including the approach described in U.S. Pat. No. 4,780,805.
As mentioned above, an objective of the present invention is to provide an improved dual input power supply of the type described above, which is simpler and more straight-forward than the power supplies of the prior art, and yet which accomplishes its desired functions in an improved and reliable manner.
The system described in U.S. Pat. No. 4,780,805 provides for the automatic configuration of a power supply by switching between either a voltage doubler or full-wave rectifier operating mode in response to the level of the AC input voltage, and the system described in the patent additionally provides a circuit configuration which locks the power supply into the full-wave rectifier operating mode once it has been switched to that mode and prevents the circuit from switching out of the full-wave rectifier operating mode in the presence of a subsequent low level input voltage so as to prevent damage to the equipment.
However, the protective circuit described in U.S. Pat. No. 4,780,805 is somewhat complex, and an additional objective of the present invention is to provide a simplified protective circuit which, after the power supply has been conditioned to the full-wave rectifier mode of operation, cannot be switched back to the voltage doubler mode of operation, unless the AC input voltage drops to a relatively low value for an appreciable time interval.